.foo
  background-image: url('https://foo.com/img/bar.png')


.foo
  background-image: url('http://foo.com/img/bar.png')


.foo
  background-image: url('//foo.com/img/bar.png')


.foo
  background-image: url('/img/bar.png')


.foo
  background-image: url('img/bar.png')


.foo
  background-image: url('bar.png')

.foo
  background-image: url('')
